Cough stress rib fractures in two obstetric patients: case report and pathophysiology.

The clinical presentation of fractured ribs and physiology of cough in two obstetric patients are described to explain why a rib fracture, not a pneumothorax, occurred on coughing in these patients. At total lung capacity the outward expansion of the lower thorax (flare) during a cough is limited. The costal angles are widened at term pregnancy, and flare may then be almost impossible at total lung capacity. The insertional action of the diaphragm is virtually nil. A direct expiratory action of abdominal muscles predominates over their indirect inspiratory action. The lower ribs are pulled down by the cough and may even fracture from the enormous intrapleural cough pressures which are generated before the glottis opens.